1. Field of the Invention
The present invention pertains in general to the field of electronic still imaging and, more particularly, to an electronic camera incorporating digital processing of image signals derived from an electronic image sensor and digital storage of the processed signals in a removable storage medium.
2. Description Relative to the Prior Art
An electronic still camera employing non-volatile storage of digital image signals is described in U.S. Pat. No. 4,489,351. Analog color information from three charge-coupled device (CCD) image sensors is converted into a digital bit stream and transmitted through a peripheral memory control unit to an integrated circuit memory. The memory is one unit of many, e.g., twenty-four memory units, recessed into a "cassette" that is separably attached through an electrical connector to the camera body. In order to obtain a digital image of high quality, many pixels, and thus many bits of digital information, need to be processed in a short time. In an article entitled "Possibilities of the Digital Electronic Still Camera", by Sumihisa Hashiguchi (Shashin Kogaku, pp. 110-111, Feb. 1988), the author proposes a multi-layer image processing integrated circuit including sensors, analog-to-digital (A/D) converters, and 8-bit buffer storage cells in respective layers. Since the output signal from an individual pixel is transferred "vertically" through an A/D converter to an included storage cell, real-time throughput is obtained without high speed operation. The stored signals can be read out slowly for digital recording, perhaps after compression, on a storage drive incorporating a small floppy disk. (Another example of a digital-based electronic still camera is shown in published UK Patent Application 2089169, in which the camera loads the digital image signals into a bubble memory cassette.)
A static random access memory (SRAM) card, in the size, and form, of a credit card, is an attractive storage alternative to the devices described in the above-related disclosures. For instance, published European Patent Application 289,944 shows a detachable SRAM module for use in a digital electronic still camera. The module is disclosed as a 32 M-bit (4 M-byte) SRAM integrated circuit card, although such storage capacities in a card are not commonly available at this time. A 512 K-byte SRAM card is presently available (Mitsubishi Electronics America, Inc. is one supplier). However, as pointed out in an article by Sumihisa Hashiguchi ("Picture Recording and Electric Power Consumption," Shashin Kogvo, pp. 94-95, Apr. 1988), there is a significant problem with memory volume. In the case, for example, of recording 780.times.490 picture elements from a CCD image sensor, with 8 bits allocated to each picture element, 382,200 bytes are required for a single monochrome video frame. This amounts to only one picture on a memory card (of 512 K-bytes). This is a considerable obstacle since still photographers are used to taking many pictures, e.g., 24 or 36 pictures, with one cassette of conventional film. Moreover, color pictures would ordinarily require three times the storage capacity of monochrome pictures.
Dynamic random access memory (DRAM) offers more storage in a reasonable volume, but power consumption quickly becomes formidable as storage capacity increases. The Hashiguchi article, consequently, calls for the development of new techniques of storage based on the compression of picture information by a factor of 10 or 100. As Hashiguchi points out, several picture compression techniques are available at present. For example, the aforementioned European Patent Application 289,944 suggests an embodiment in which a signal processor is adapted to accomplish data compression, such as the Hadamard transform, cosine transform or orthogonal transform, and coding on the video signal, which in turn is transferred to and stored interest that U.S. Pat. No. 4,131,919, which issued on Dec. 26, 1978, proposes the use of source and/or channel encoding schemes to more efficiently record digital still image signals on magnetic tape.) Adaptive differential pulse code modulation is another known compression algorithm for encoding still images.
The fundamental structure for in-camera digital processing is ordinarily based on a conventional analog camera, with digital processing techniques being applied to the functional analog blocks, such as color separation, white balance, gamma correction, and so on. This conventional transposition extends to real-time processing in that in-camera digital processing seeks, insofar as possible, to emulate real-time analog processing rates by rapidly accessing the imager, processing the resultant image signals, and writing the processed image signals to memory within normal video frame rates. (. . . albeit, that in the aforementioned Shashin Kogaku article of Feb. 1988, in U.S. Pat. No. 4,489,351, and in UK Patent Application 2089169, a buffer or temporary memory is provided to allow transmission of the image data to the recording device at a desired rate, which due to device or other limitations is often less than the image capture rate.) Nonetheless, as recognized by the Hashiguchi article, the available techniques neither adequately meet the requirement for real-time processing as needed by an electronic still camera nor the requirement for simply including the compression hardware with the camera.